Although mechanically compensated zoom systems for visible radiation working at a very low percentage of diffraction cut off are common place, infra-red zoom lenses are rare because they require a high state of correction (in order to work near to the diffraction limit at high spatial frequencies) and because of the problems associated with materials at this wavelength. Consequently, the diffraction limited 4:1 thermal imaging zoom system developed by PPE represented a significant optical design achievement. The continuation of this work has been to develop a 3:1 ultra compact thermal imaging zoom telescope to be built in production quantities to a severe environmental specification. The first prototypes have now been built and development work is being carried out in a number of areas. The opto-mechanical tolerancing is discussed with particular reference to the problem of volume production.